How much time are you spending looking at a display each day? From smartphones, tablets, and laptops to televisions, displays have become an indispensable part of our daily lives. Not just limited to personal devices, but modern innovations, such as kiosks, virtual reality (VR) and augmented reality (AR) are growing immensely, further expanding the role of display in our future. The two most dominant displays currently are LCD and OLED. OLED, in particular, may be more familiar to us, as the brand of our televisions is named after OLED. OLEDs have self-emitting properties where each pixel independently emits light when an electrical current transports through it. The independent control of brightness and color in each pixel allows for excellent black levels and contrast ratios.

On the other hand, LCDs are the primary models of our displays using a consistent backlight regardless of the displayed content. Thus, through the development of OLEDs, we now have our current displays with deeper black levels, lower power consumption, and high color purity, available in small wearable devices to large-area screens. OLEDs can also be fabricated on flexible substrates, highlighting their high compatibility with next-generation flexible and foldable displays. So, what is next in OLEDs?

OLEDs comprise three main sections: the light-emitting layer, the thin-film transistor (TFT) that switches and drives the electrical signals to make the light-emitting layer produce light and the substrate. The part where TFTs are fabricated on top of the substrates is called the backplane. OLEDs heavily rely on advanced backplane technology to control the functionality of each pixel, ranging from the image quality, and power consumption, to other fundamental features of the display.

Previously, LCDs used amorphous silicon (a-Si) as a backplane TFT semiconductor. However, due to its low charge carrier mobility, the speed and amplitude of pixels to turn on and off were limited. Next, low-temperature poly-silicon (LTPS) was developed, which has high electron mobility compared to a-Si and has more efficient pixel switching. LTPS also has its limitations with high processing costs. LTPS requires an excimer laser annealing (ELA) process for crystallization, which is not only expensive but also has limited beam size, which causes a challenge when applying for large-scale fabrication. Thus, a novel class of oxide semiconductors has been developed to compensate for the low mobility of a-Si and the high cost of LTPS. Metal oxide semiconductors are composed of ionic materials where the ns orbital of the metal and 2p orbital of the oxygen form their conduction band minimum (CBM) and valence band maximum (VBM), respectively. This leads to forming a wide bandgap, which significantly reduces the leakage current when the display is turned on and reduces power consumption. The most successful commercialized metal oxide is indium gallium zinc oxide (IGZO), offering high electron mobility for driving TFTs, using cost-effective materials and fabrication techniques, and allowing uniform amorphous thin-film deposition over large areas. Therefore, IGZO has successfully been incorporated into OLED backplanes. 

Nevertheless, oxide semiconductors have a critical drawback. Like IGZO, most metal oxides have high electron mobility, but very low hole mobility at room temperature, offering a good n-type semiconductor, but a very poor p-type semiconductor. For broader application of oxide semiconductors, the development of high mobility p-type oxide semiconductors is required. Developing a high mobility p-type oxide is intrinsically challenging due to the difference in properties of CBM and VBM, which determine the electron and hole transport, respectively. The ns orbitals of the metals form spherical CBM, allowing facile electron transport over a large overlap between orbitals. In contrast, the 2p orbitals of oxygen are localized, limiting the efficiency of hole transport. Thus, the development of high-performance amorphous p-type semiconductors has been the most difficult challenge that the semiconductor community has faced over the past two decades. 

We have opened the door to solve this ‘impossible’ task, by introducing a novel material called tellurium oxide. Tellurium (Te) is a chalcogen heavy metal from Group 16 on the periodic table. We have taken a unique approach to use a chalcogen heavy metal for p-type oxides, instead of the traditional metal cations. There are two main steps to the material engineering of tellurium oxides. First, by creating a partially oxygen-deficient state in tellurium oxide, Te2⁺, and Te0 states are produced, forming an acceptor level near the VBM to take in hole carriers. We are able to boost the hole carrier concentration by adjusting the oxidation of the partially oxidized tellurium oxide. Next, selenium (Se) is introduced with tellurium oxide to form tellurium-selenium composite oxide. Te-Se alloy can be used as a conduction channel for hole transport, creating highly efficient p-type oxide semiconductors. Finally, Se-alloyed tellurium oxide can be used to fabricate high-performance amorphous p-type TFT, reaching the performance parameters comparable to amorphous IGZO TFTs.

Se-alloyed tellurium oxide TFTs introduce superior performance and device stability compared to previously reported p-type oxide semiconductors. The Se-alloyed tellurium oxide thin-film can be uniformly fabricated over large areas at low processing temperatures with low production cost, easily applicable to diverse electronic devices. We continue to explore various semiconductors and apply novel materials to increase the efficiency and lower power consumption of next-generation electronic devices and circuits.